1. Field of the Invention
The present invention is directed generally to a bipolar transistor which is controllable by field effect, and more particularly to such transistor having a semiconductor body with an inner zone and a zone at a drain side which has a higher doping than the inner zone and is of an opposite conductivity type.
2. Description of the Related Art
Bipolar transistors of the type related to the present invention are described, for example, in an article entitled, "COMFET-The Ultimate Power Device" in the periodical SOLID STATE TECHNOLOGY, Nov. 1985, pages 121-128. The disclosed device has a source side which is formed like a power MOSFET; however, at its anode side, the device has a fourth zone which is of a conductivity type opposite to that of the inner zone. The device, thus, has a structure like that of a thyristor and is provided with a shunt at a cathode side which increases what is referred to as the latching current of the thyristor so that the latching current does not reach the operating conditions of the component. As in a thyristor, but by contrast to the operation of a power MOSFET, both types of charge carriers participate in current transportation in the disclosed device. On one hand, this produces the advantage of a low on-state dc resistance, while, on the other hand, it yields the disadvantage of recovery, charge which is felt in what is referred to as a "tail" current during shut-off. Such tail current delays shutoff of the device.
According to various proposals, the recovery charge is reduced by, for example, recombination centers or by imperfections or damage in the lattice structure caused by irradiation. A further proposal is to insert a buffer zone between the inner zone and the anode zone. The buffer zone is of the same conductivity type as the inner zone but has a higher doping concentration than the inner zone (see, for example, the chapter entitled, "Power Control" in the periodical ELECTRONIC DESIGN, 12 Jan. 1984, pages 248 and 249).
It is possible to reduce the recovery charge and, thus, to diminish a shut-off time with the above described measures or with a combination of both such measures.